The invention relates generally to apparatus and methods for using frictional drives including conformable members in electrostatography, and more particularly to the use of frictional drives for transferring and fusing toner images in electrophotography.
During the production of color images in an electrostatographic engine in general and in an electrophotographic engine in particular, latent images on photoconductive surfaces are developed by electrostatic attraction of triboelectrically charged colored marking toners. A latent image is created in a color electrophotographic engine by exposing a charged photoconductor (PC) using, for example, a laser beam or LED writer. A plurality of toner images correspond to color separations that will make up a final color image. Individual writing of the color separation latent images must be properly timed so that the various latent images developed from the latent images can be transferred in registry. The toned image separations must then be transferred, in register, to either a receiver or to an intermediate transfer member (ITM). The toner images can be transferred, either sequentially from a plurality of photoconductive elements to a common receiver in proper register, or transferred, sequentially, in proper register, to one or more ITMs from which all images are then transferred to a receiver. Alternately, each photoconductive surface may be associated with its own ITM, which transfers its toned image, in proper register with those of the other ITMs, to a receiver, for the purpose of enhancing the transfer efficiencies as more fully described in T. Tombs et al., U.S. Pat. No. 6,075,965. A toner image on the receiver is thermally fused in a fusing station, typically by passing the receiver through a pressure nip which includes a heated fuser roller and a pressure roller.
A key feature is that transfers must be performed in proper registry. The degree of misregistration that can be tolerated in an acceptable print depends on the image quality specifications. For high image quality color applications, allowable misregistration is typically less than 0.004 inch (0.1 mm) and preferably less than 0.001 inch (0.025 mm). Misregistration is often examined using 10xc3x97 to 20xc3x97 loupes to determine relative positions of interpenetrating fiducial line or rosette patterns. In systems involving elastomeric rollers and in particular in machines including compliant incompressible elastomeric rollers as intermediate transfer members, as described by D. Rimai et al., U.S. Pat. No. 5,084,735, the rollers are known to deform as they roll under pressure against a photoconductive surface which may include a web or a drum. These intermediate transfer members also undergo deformations as they roll against receiver materials either as continuous webs or as cut sheets that can be supported by a web or by a backup roller assembly, or by combinations of these. Other prior art disclosing ITMs include U.S. Pat. Nos. 5,110,702; 5,187,526; 5,666,193 and 5,689,787.
Deformation of a conformable member produces a phenomenon known as overdrive. Overdrive refers to the fact that in a nip including an elastomeric roller in mutual nonslip rolling engagement with a relatively rigid roller, the surface speed of the rigid roller exceeds the surface speed of that portion of the elastomeric roller that is far from the nip. Far away from the nip means at a location where any distortions caused by the nip are negligible. The difference in peripheral speeds far from the nip is a result of the strains occurring in the elastomeric roller surface as it approaches and enters the nip.
The concept of overdrive may be better understood by referring to the sketches in FIGS. 1-3.
In FIG. 1, a rigid cylindrical wheel or roller is driven without overdrive. In such an example, each point on the periphery has a velocity v0 given by the product of the angular velocity xcfx89 and the radius r of the roller, i.e., v0=xcfx89r.
In FIG. 2, a deformable externally driven roller is illustrated. The deformation illustration is exaggerated to facilitate explanation of the concept that when a substantially incompressible compliant member is in a transfer nip, for example, a deformation will occur that causes the radius to be smaller in the nip area but to bulge out at pre-nip and post-nip areas. The dotted line shows the original circular rigid case of FIG. 1 for comparison. The relationship of v0=xcfx89r still holds true for points on the roller far from the nip area where there is no deformation. However, this relationship is not true for the points in the pre-nip, nip and post-nip areas. For the roller illustrated in FIG. 2 the speed of a point in the nip area has a higher magnitude than that far from the nip. The speed ratio of the roller surface in the nip divided by the speed at a point far from the nip area characterizes overdrive.
More particularly consider, for example, a conformable roller having an externally driven axle, frictionally driving with negligible drag a movable planar element having a nondeformable surface. If the external radius of the roller far from the nip is r and the peripheral speed of the roller far from the nip is v0, then the surface velocity vnip of the distorted portion of the roller in nonslip contact with the planar surface is given by
vnip=xcexxcfx89r
where xcex is a speed ratio defined by
xcex=(vnip/v0).
As defined here, overdrive (or underdrive) is numerically equal to the absolute value of the speed ratio minus one. The value of xcex is determined principally by an effective Poisson ratio of the roller materials, such as produced by a roller including one or more layers of different materials, and secondarily, by the deformation geometry of the nip produced by the roller engagement. Herein, the term engagement, in reference to a pressure nip formed between two members having operational surfaces, is defined as a nominal total distance the two members are moved towards one another to form the nip, starting from an initial undeformed, barely touching or nominal contact of the operational surfaces. In FIG. 3a or 3b, for example, the engagement is the distance the axis of rotation of the roller is moved towards the rigid planar element from a nominal initial kissing position. In an example of two parallel rollers, the engagement is an initial separation of the two axes of rotation (defined by a nominal initial kissing position with neither roller distorted) minus the actual separation of the axes after the nip is formed.
The Poisson ratios of high polymers, including elastomeric polymers which for practical purposes are almost incompressible, approach 0.5. The Poisson ratios for highly compressible soft polymeric foams approach zero. It has been shown by K. D. Stack, xe2x80x9cNonlinear Finite Element Model of Axial Variation in Nip Mechanics with Application to Conical Rollersxe2x80x9d (Ph.D. Thesis, University of Rochester, Rochester, N.Y. (1995), FIGS. 5-6 and 5-7, pages 81 and 83) that the value of Poisson ratio for xcex=1 is about 0.3 for a roller driving a rigid planar element. For values of Poisson ratio larger than about 0.3, the circumference of the roller distorted by the nip is greater than 2xcfx80r, producing overdrive of the planar element with respect to the roller, i.e., the surface speed vnip of the distorted portion of the elastomeric roller within the nip and hence that of the planar element is greater than v0 (i.e., xcex greater than 1). For values of Poisson ratio smaller than about 0.3, the circumference of the elastomeric roller distorted by the nip is less than 2xcfx80r, producing underdrive of the planar element with respect to the roller, i.e., the surface speed vnip within the nip is smaller than v0 (i.e., xcex less than 1). Conversely, if a nondeformable planar element frictionally drives, with negligible drag, a roller having a Poisson ratio less than about 0.3 and causes it to rotate, one may speak of overdrive of the roller with respect to the planar element because the surface speed of the driven roller far from the nip is faster than the speed of the planar element.
With reference to FIG. 3b, when a roller transfer member formed of an elastomer and having a Poisson ratio of about 0.45 to about 0.5 is driving a rigid planar element that is moving through a nip and there is no slippage between the roller and the rigid element, the rigid element will be overdriven relative to the speed of the roller far from the nip. Where the roller is formed of a compressible material (i.e., experiences relatively large volume reduction upon compression), such as a foam, the distortion of the roller may be such (see FIG. 3a) that the surface of the roller is contracted rather than stretched. Compare FIG. 3a with the example of the elastomeric roller of FIG. 3b having little or no volume change upon compression, with each roller shown in driving engagement with a rigid planar element. In the example of the highly compressible roller (relatively large volume change upon compression) of FIG. 3a, the rigid planar element such as a recording sheet may be subject to an underdrive condition.
For purpose of further illustration, FIG. 3c illustrates an exemplary apparatus, indicated by the numeral 5, which includes two counter-rotating rollers 1 and 2 forming a pressure nip 3. Far away from the nip, rollers 1 and 2 have peripheral speeds v1 and v2 respectively. Roller 2 is hard, and roller 1 is conformable, with roller 1 having a strained volume portion sketched by a cross-hatched region 4 in the vicinity of the nip (deformation of the surface of roller 1 is not depicted). Consider that one of the axles P or Q is caused to rotate by the action of an external agent, such as for example a motor, and the other axle is rotated by nonslip friction in the nip. The externally rotated roller is a driving roller, while the other is a (frictionally) driven roller. There are four extreme cases to consider. Case 1: roller 1 is the driving roller, and region 4 is a substantially incompressible elastomer, whereupon as explained above the peripheral velocity v2 of roller 2 far from the nip is greater than the peripheral velocity v1 of roller 1 far from the nip, and roller 2 is said to be overdriven. Case 2: the same materials as case 1, except that roller 2 is the driving roller and roller 1 is the driven roller, whereupon roller 1 is said to be underdriven. Case 3: roller 1 is the driving roller, and region 4 is a compressible resilient foam, whereupon the peripheral velocity v2 of roller 2 far from the nip is smaller than the peripheral velocity v1 of roller 1 far from the nip, and roller 2 is said to be underdriven. Case 4: the same materials as case 3, except that roller 2 is the driving roller and roller 1 is the driven roller, whereupon roller 1 is said to be overdriven. It should be noted that it is common practice to use the term xe2x80x9coverdrivexe2x80x9d in a generic or nonspecific fashion where either overdrive or underdrive technically exists.
Two materials in contact in a pressure nip may have different thicknesses or different Poisson ratios, so that overdrive at their interface can cause squirming and undesirable stick-slip behavior. For example, when roller transfer members are used to make a color print, such behavior can adversely affect the final image quality, e.g., by causing toner smear or by degrading the mutual registration of color separation images. Moreover, variations in overdrive, which are referred to herein as xe2x80x9cdifferential overdrivexe2x80x9d can occur along the length of a pressure nip, such variations being caused, for example, by local changes in engagement, such as produced by runout, or by a lack of parallelism, or by variations of dimensions of the members forming a pressure nip, such as, for example, out-of-round rollers.
During transfer of a toner image in an elastomeric nip exhibiting overdrive or underdrive, an image experiences a length change in the process direction. This change in length causes a distortion in the final image that is objectionable. Change in the writing speed of an electrostatic latent image can correct for overdrive in a simple single-color engine. In a color electrophotographic engine, however, high quality color separations preferably are properly registered to a spatial accuracy comparable with the resolution of the image. In a color electrophotographic engine including a plurality of color stations, proper registration can be achieved by having each color station behave exactly in the same manner with respect to image distortion, e.g. by using rollers made as identical as possible to each other. However, this is expensive and impractical.
Specifically, in order to produce proper electrophotographic images using techniques of the prior art, properties of rollers must not vary outside predetermined acceptable tolerances. The properties include acceptable runout, reproducible and uniform resistivity and dielectric properties, uniform layer thicknesses, parallelism of the members, and responses of the rollers to changes in temperature and humidity experienced during routine operation and machine warm-up. Rollers must also maintain their properties within tolerances during wear processes so that adverse effects are not experienced on the final images as a result of wear. If the effects of wear cannot be compensated, the components must be replaced.
A roller may have variations in the location of the roller surface relative to the roller center as a function of angle during rotation that is commonly known as xe2x80x9crunoutxe2x80x9d. Runout may be caused by out of round rollers or by improper centering of an otherwise round roller or both. Runout may vary along the length of a roller. Since the magnitude of the overdrive produced by a deformable roller depends on engagement, runout will temporally and spatially modify the engagement and overdrive during the production of a single image, producing distortions that are objectionable. Runouts of 0.001 inch (0.025 mm) can produce unacceptable registration problems, with runouts of less than 0.0002 inch (0.05 mm) needed to achieve acceptable registration based on measured sensitivity of overdrive to engagement.
Further, rollers used in these applications are made from polymers that can change dimension by absorption of moisture and can change dimensions due to temperature changes. These dimensional changes further complicate the registration of color separations if the changes are not the same in each of the color separation stations included in a color electrostatographic engine.
Methods based on the prior art to produce a workable electrophotographic engine with useful image quality require very expensive manufacturing processes to control the properties and dimensions of the elastomeric rollers.
What is needed is an improved method to alleviate or effectively eliminate image distortion caused by overdrive or underdrive phenomena. As is known, this can be performed by expensive algorithms to the writing scheme using sensors to detect surface speeds of elements during writing and transfer.
There are several disclosures in the prior art that relate to the peripheral speeds of rollers. The T. Miyamoto et al. patent (U.S. Pat. No. 5,519,475) teaches the use of peripheral speed differences between a photoconductive member and an intermediate transfer member (ITM) to reduce the apparent roughness of the surface. The Miyamoto et al. patent describes transfers from the photoconductive members to transfer intermediates where there is a peripheral speed difference of 0.5% to 3%. The K. Tanigawa et al. patent (U.S. Pat. No. 5,438,398) includes disclosure relating to peripheral speeds. In particular, embodiments 6 and 7 of this patent suggest that an intentional peripheral speed difference of 1% helps with xe2x80x9ccentral dropoutxe2x80x9d defects. The patent notes that transfers of images are intentionally provided with differences in peripheral speeds, but no description is provided relative to overdrive or underdrive as described herein. Another known reference is the M. Yamahata et al. patent (U.S. Pat. No. 5,390,010). This patent specifically addresses the behavior of web photoconductors (PCs) and web ITMs with the central idea to use the same drive motor to drive an intermediate transfer web drive roller which in turn drives the web drive roller of a photoconductive web. Thus, disturbances in surface speed of the ITM web, such as might be caused by engagement of a cleaning station, etc., would be transmitted to the PC web so that there would not be image degradation due to slippage. The Yamahata et al. patent does not discuss how this would affect the writing of an image. There is no disclosure in this patent of transfers where a nip is formed by an elastomeric member and the problems of overdrive or underdrive as it affects image registration. It is clear that this reference addresses the problem of slippage of the ITM relative to the PC when such slippage is caused by disturbances of the system.
The T. Fuchiwaki patent (U.S. Pat. No. 5,790,930) discloses a means for correcting for misregistration between an image-carrying member and an intermediate transfer web due to variations in the length of the two members. It accomplishes this by means of forcing a periodicity in the drive speeds. It can achieve this by means of either two motors or a single motor.
The S. Hwang patent (U.S. Pat. No. 5,376,999) discloses a method of correcting for speed mismatches between a photoconductive element and an intermediate transfer web due to the stretching of that web arising from the tension applied to that web. The strains described in this patent occur outside the nip. The patent discloses allowing one member to slip with respect to the other where both members are driven. There is no discussion of an elastomeric intermediate transfer member in this patent. In an elastomeric intermediate transfer member, the distortions occur due to the presence of stresses applied normally to the surface of the elastomeric member in the nip rather than due to stresses applied parallel to the surface of the elastomeric member.
Problems relating to overdrive are also typically found in fusing stations used in electrostatographic imaging and recording processes such as electrophotographic reproduction, in which a thermoplastic toner powder is used to form a toner image on a receiver, e.g., a sheet of paper or plastic. The toner image is fused to the receiver in a fusing station using heat or pressure, or both heat and pressure. The fuser member can be a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver. The fusing step in a roller fuser commonly consists of passing the toned receiver between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form such nip, at least one of the rollers typically has a compliant or conformable layer on its surface. Heat is transferred from at least one of the rollers to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver. In the case where the fuser member is a heated roller, a resilient compliant layer having a smooth surface is typically used. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, it typically has a smooth, hardened outer surface. A belt fuser of this type is well known, as disclosed for example by the Aslam et al. patent (U.S. Pat. No. 5,256,507) wherein the belt is driven by the fuser roller, the belt in turn frictionally rotating a pressure roller which forms a fusing nip between itself and the heated roller behind the belt. Other disclosures of fusing stations utilizing a belt are the Goel et al. patent (U.S. Pat. No. 3,976,370), the Rimai et al. patent (U.S. Pat. No. 5,089,363), and the Aslam et al. patent, (U.S. Pat. No. 5,258,256).
Most roller fusers, known as simplex fusers, attach toner to only one side of the receiver at a time. In this type of fuser, the roller that contacts the unfused toner is commonly known as the fuser roller and is usually the heated roller. The roller that contacts the other side of the receiver is known as the pressure roller and is usually unheated. Either or both rollers can have a compliant layer on or near the surface. In most fusing stations including a fuser roller and an engaged pressure roller, it is common for only one of the two rollers to be driven rotatably by an external source. The other roller is then driven rotatably by frictional contact.
In a duplex fusing station, which is less common, two toner images are simultaneously attached, one to each side of a receiver passing through a fusing nip. In such a duplex fusing station there is no real distinction between fuser roller and pressure roller, both rollers performing similar functions, i.e., providing heat and pressure.
Two basic types of simplex heated roller fusers have evolved. One uses a conformable or compliant pressure roller to form the fusing nip against a hard fuser roller, such as in a Docutech 135 machine made by the Xerox Corporation. The other uses a compliant fuser roller to form the nip against a hard or relatively non-conformable pressure roller, such as in a Digimaster 9110 machine made by Heidelberg Digital LLC. A fuser roller designated herein as compliant typically includes a conformable layer having a thickness greater than about 2 mm and in some cases exceeding 25 mm. A fuser roller designated herein as hard includes a rigid cylinder which may have a relatively thin polymeric or conformable elastomeric coating, typically less than about 1.25 mm thick. A fuser roller used in conjunction with a hard pressure roller tends to provide easier release of a receiver from the heated fuser roller, because the distorted shape of the compliant surface in the nip tends to bend the receiver towards the relatively non-conformable pressure roller and away from the much more conformable fuser roller.
A conventional toner fuser roller includes a cylindrical core member, often metallic such as aluminum, covered by one or more synthetic layers which typically include polymeric materials made from elastomers.
In an internally heated fuser roller, e.g., as used in a Kodak Ektaprint 3100 Copier/Duplicator and the Kodak 1392 Printer, a source of heat is provided within the roller for fusing. Such a fuser roller normally has a hollow core, inside of which is located a heating source, usually a lamp. Surrounding the core is an elastomeric layer through which heat is conducted from the core to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity.
An externally heated fuser roller is used, for example, in an Image Source 120 copier, marketed by Eastman Kodak Company, and is heated by surface contact between the fuser roller and one or more heating rollers. Externally heated fuser rollers are also disclosed by the O""Leary patent (U.S. Pat. No. 5,450,183), and the Derimiggio et al. patent (U.S. Pat. No. 4,984,027).
A conformable fuser roller may include a compliant layer of any useful material, such as for example a substantially incompressible elastomer, i.e., the layer having a Poisson ratio approaching 0.5. A substantially incompressible compliant layer including a poly(dimethyl siloxane) elastomer has been disclosed by Chen et al., in the commonly assigned U.S. patent application Ser. No. 08/879,896. Alternatively, the conformable layer may include a relatively compressible resilient foam having a value of Poisson ratio much lower than 0.5. A conformable polyimide foam layer is disclosed by the Lee patent (U.S. Pat. No. 4,791,275). Generally speaking, a conformable or deformable material or roller is defined hereinafter as including compliant materials such as elastomeric materials, or resilient foams.
When a compliant fuser roller and a hard pressure roller which are included in a simplex fusing station are pressed against each other, the compliant layer is deformed and is peripherally stretched in the fusing nip, causing the surface speed of the portion of the compliant roller having a nonslip engagement inside the nip to be faster than the surface speed where distortions produced by the nip are negligible. When, for example, the compliant roller is a driving roller frictionally rotating a relatively non-conformable pressure roller, the pressure roller will rotate faster than if the fuser roller had been non-compliant, i.e., it will be overdriven as discussed previously above (see description of FIGS. 1, 2 and 3). Hereinafter, the terms xe2x80x9chardxe2x80x9d and xe2x80x9cnon-conformablexe2x80x9d are used interchangeably, and refer to materials for which the Young""s modulus is greater than or equal to 100 MPa.
A substantially incompressible elastomer that is displaced in the fusing nip results in an extra thickness of the compliant layer adjacent to either side of the fusing nip, i.e., pre-nip and post-nip bulges. Since the elastomer is substantially incompressible, the average speed of the compliant layer in these bulges is less than that of the other parts of the conformable layer that are well away from the nip. It may be understood that to produce a frictional drive involving a conformable roller, there is a xe2x80x9clockdownxe2x80x9d portion within the contact zone of the nip where there is substantially no slippage between the driving and driven members. Moreover, during the continual formation and relaxation of the pre-nip and post-nip bulges or deformations on the roller as it rotates through the fusing nip, there may be locations in the contact zone of the nip where the surface velocities of the two surfaces in contact differ, i.e., there will be localized slippages. These localized slippages, which may occur just after entry and just before exit of the nip, are a cause of wear which shortens roller life. In order to avoid confusion below, a frictional drive is hereinafter defined as being nonslip if a xe2x80x9clockdownxe2x80x9d region exists in the nip wherein the coefficient of friction is sufficiently large to provide a continuous frictional driving linkage between the contacting members within the nip. This definition excludes any localized slippages that may occur in the contact areas near the entry and exit of the nip, because these localized slippages are in opposite directions and any effects on the drive produced by them effectively cancel. In other words, the frictional linkage in the xe2x80x9clockdownxe2x80x9d portion is the only factor of importance in determining a driving connection produced by the nip. Hereafter, the words xe2x80x9cnonslipxe2x80x9d, xe2x80x9cslipxe2x80x9d and xe2x80x9cslippagexe2x80x9d refer to an externally measured behavior of the members involved in the frictional drive, e.g., as described below in the specification of the present invention.
All rollers suffer from surface wear, especially where the edges of receivers contact the rollers. Since relative motion due to slippage between rollers increases wear, the changes in velocity of the surface of a conformable roller, as it travels into, through, and out of a fusing nip formed with a relatively non-conformable roller, should increase the wear rate of the conformable roller, especially if the conformable roller is the heated fusing member, bearing in mind that a fuser roller typically faces a relatively rough and abrasive paper surface in the nip.
To obtain high quality electrophotographic copier/printer image quality, image defects must be reduced. One type of defect, of particular importance in high quality digital color imaging, is produced by smearing of image dots or other small-scale image features in the fusing nip. Relative motions associated with overdrive, e.g., localized slippage between rollers in a fusing nip, can cause softened toner particles to smear parallel to the direction of motion, resulting for example in elongated dots or blurred edges in an image. Such defects can make a color print unacceptable.
Some roller fusers rely on film splitting of low viscosity oil to enable release of the toner and (hence) receiver from the fuser roller. Relative motion in the fusing nip can disadvantageously disrupt the oil film. This may be acute when fusing a 4-color toner image which requires more fuser oil than a black and white image. An increased amount of fuser oil also increases any tendency for slippage.
Image gloss from a roller fuser is more critically dependent upon the time a toned receiver is in the fusing nip than is the fuser nip pressure. Thus, fuser nip width is a critical parameter and is more important than the nip engagement or load, especially for fusing full color images where the toner stack height is much greater than for a black and white toner image. To rival the glossiness of silver halide technology prints, it is desirable that multicolor toner images have high gloss. To this end, it is desirable to provide a very smooth fusing member contacting the toner particles in the fusing station.
In the fusing of the toner image to the receiver, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver sheet as it passes through the fusing nip is determined by the amount pressure exerted by the pressure roller and by the characteristics of the resilient cushion layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller.
A well known problem in fusing is that paper receiver sheets may not be perfectly rectangular, in part as a result of humidity-induced swelling. After manufacture, paper sheets are typically stacked and conditioned in a humidity controlled environment. During this time, moisture partially penetrates the paper through the edges of the sheets. For typical commercial paper used in electrophotographic machines, moisture penetration is much faster in a direction parallel to the orientation of the long paper fibers. A typical 8.5xe2x80x3xc3x9711xe2x80x3 paper sheet has long paper fibers oriented substantially parallel to the 11xe2x80x3 direction, and moisture therefore penetrates preferentially into the 8.5xe2x80x3 edges. This causes the nominally 8.5xe2x80x3 edges to expand, so that the 8.5xe2x80x3 edges become about 1% to 2% longer than the width of the paper measured across the center of the sheet (parallel to the 11xe2x80x3 direction). It is usual practice to feed such paper sheets into a fuser nip with the 8.5xe2x80x3 edges parallel to the feeding direction, i.e., perpendicular to the roller axes. Therefore, unless corrective measures are taken, it typically takes a longer time for the swollen 8.5xe2x80x3 edges to pass through the fusing nip than it does for the middle of the sheet, which can result in severe paper wrinkling and large scale image defects. In order to provide a correction for this problem, it is known that elastomerically coated fusing station rollers may be manufactured with an axially varying profile obtained by gradually varying the thickness of the elastomeric coating, such that the outer diameter of a roller is greater near the ends of the roller than midway along the length of the roller. Inasmuch as elastomerically induced overdrive increases with increasing engagement, the larger engagements nearer the ends of the roller produce locally larger surface velocities of the paper through the nip, thereby tending to compensate for humidity-induced paper swelling by having all portions of the paper spend substantially the same time passing through the nip. As is also well known, a pressure nip formed between two rollers, at least one of which has an elastomeric coating, does not usually have a uniform pressure distribution measured in the axial direction along the length of the rollers. Rather, owing to the fact that the compressive forces are applied at the ends of the rollers, e.g., to the roller axle, the rollers tend to bow outwards slightly, thereby producing a higher pressure near the ends of the rollers than midway along their length. This also tends to produce greater overdrive towards the ends of the rollers. However, the amount of extra overdrive from roller bending is not normally sufficient to compensate for humidity-induced paper swelling, and therefore a profiling of the thickness of the elastomeric coating in the axial direction, as described above, is often practiced.
To improve image quality of a fused toner image, and also to reduce wear and aging and thereby prolong the life of a conformable roller in a fusing station, there remains a need for inexpensive means to control or eliminate overdrive-induced wear of the roller. There also remains a need to prevent or reduce overdrive-induced image defects, either large-scale or small-scale, when using a conformable roller in a fusing station.
In electrostatography in general and, more particularly in electrophotography, the elimination of overdrive or underdrive in a conformable nip is desirable because overdrive and variations in overdrive can cause image defects such as misregistration of color separation images objectionable to the customer. There is a need to provide a simple, inexpensive mechanism to control or eliminate overdrive related registration artifacts.
An important aspect of this invention includes a method and apparatus to control image defects related to transfer of toner images in an electrostatographic machine, including defects such as misregistration associated with overdrive or underdrive and variations in overdrive and underdrive in a transfer station including a toner image bearing member. In this aspect of the invention, a speed modifying force is applied to a conformable transfer member that forms a nip for transfer of an image, thereby inducing strains in the surface of the member at the nip which will cancel or controllably reduce the strains caused by the engagement of the conformable nip. This lateral force, which is directed along the direction of motion in the nip, may be an externally applied drag force such as for example a friction force that either opposes motion of the elements engaged at the nip (positive drag), or of the opposite sign which urges faster motion of the elements engaged at the nip (negative drag), and may be applied using an open loop or a feedback system including an electromagnetic brake, a motor, etc. (Note that any system involving one or more pressure nips will generally have an inherent drag, e.g., due to friction, which is to be distinguished from an applied drag force of the invention). Alternatively, the speed modifying force may be produced by a controllable torque applied for example by a torque generator to an axle of a roller included in a frictionally driven system of rollers. In a preferred embodiment, the speed modifying force is applied to an elastomeric member forming the nip through a redundant linkage of the system that employs gears or other suitable mechanisms. In this latter case, the action of a frictionally engaged nip with its overdrive working against a redundant mechanical linkage will cause a drag force to develop which is of precisely the correct sign and magnitude to cancel the surface strain responsible for the overdrive normally produced by the frictional engagement of the operational surfaces of the members forming the nip. A transfer system according to the present invention may have a steady state overdrive or underdrive, including the possibility of zero overdrive. The control of overdrive or underdrive is preferably independent of the extent of engagement and detailed material properties.
Another aspect of this invention includes a similar method and apparatus for providing, in a station for thermal fusing of toner images in an electrostatographic machine, a speed modifying force controllably applied to a drivingly and frictionally moved member included in a fusing nip, the fusing nip utilizing a conformable roller. The speed modifying force, which may be produced by a drag or torque, is controllably applied to reduce wear of the conformable roller and also to control image defects related to thermal fusing of toner images, such as image smear including the smearing of halftone dots. A fusing system according to the present invention preferably has a negligible or zero amount of overdrive or underdrive in the fusing nip, and the control of overdrive or underdrive in the fusing nip is preferably independent of the extent of engagement and detailed material properties.